1. Field of the Invention
The present invention relates to tantalum and niobium capacitors. More specifically the present invention is related to lead frames for tantalum and niobium capacitors.
2. Description of Related Art
In the manufacture of passive integrated circuit devices, lead frames are used to provide electrical interconnection to a semiconductor circuit. Lead frames are small metal articles that are employed to connect integrated circuits to devices in which they are used. For example, in the manufacture of a Tantalum or Niobium capacitor, the lead frame provides the physical and electrical connection from the Tantalum/Niobium capacitor pellet to the electronic circuit to which it is applied.
Another of the primary functions provided by the lead frame is to support the integrated circuit. For a Tantalum or Niobium capacitor, the lead frame provides the handling platform during the process of manufacturing the capacitor from assembly to packaging. In addition to supporting the integrated circuit, the lead frame provides connections through the encapsulation envelope. These connections are used to attach the integrated circuit (capacitor) to the device in which it performs. The Ta/Nb pellet is electrically connected to the lead frame, usually by resistance or laser welding of the Ta/Nb anode wire to the lead frame for the positive connection and with a Silver Loaded Epoxy between the cathode to the leadframe for the negative connection. After being electrically connected to the lead frame, the integrated circuit (capacitor pellet) is encapsulated with the leads on the lead frame extending through, the plastic encapsulant.
To perform optimally, the lead frame must be made of material having specific properties including one or more of following depending on the specific use: good electric conductivity which is necessary to transmit electrical impulses to and from the integrated circuit sufficient for the desired circuit/device performance; minimal thermal conductivity to prevent heat from board mounting from transmitting to the Ta/Nb pellet and damaging the dielectric or cathode material; strength and rigidity sufficient for the manufacturing handling process and to maintain it in the device in which it is employed; sufficient ductility to be capable of accepting precision forming the complex shape of the capacitor component while retaining sufficient strength to retain the component on the circuit board. For certain types of capacitors, certain properties are required. For example, for Tantalum and Niobium capacitors, lead frames must have the ability to weld to the Ta/Nb lead wire, the ability to be plated with the necessary barrier and final plating layers for attachment to the Ta/Nb pellet and to the circuit board during final application, and thermal expansion consistent with process design.
Lead frames must also be made of material that has good corrosion resistance to prevent corrosion during the fabrication stages of the capacitor and during the application on the circuit board. Tantalum or Niobium lead frames must also be low cost as capacitors are inexpensive, commodity electronic components.
Finding a material with all of these properties is difficult, and, as a result, on a commercial scale compromises are made.
To achieve all of these requirements, the Tantalum Capacitor lead frame typically consists of base metal, a plated barrier layer(s) between the base metal and final plated outer layer(s). Typical lead frame base materials used include Copper, Copper based alloys, Copper/Nickel and/or Nickel based alloys that are typically from about 0.003″ to 0.010″ thick (as opposed to about 0.010 to 0.015 for active capacitors). A barrier or mid-layer is often plated to the base material to allow suitable attachment of the Tantalum pellet. Typical barrier layers consist of Nickel, Copper and/or Silver that are approximately 2-50 micro inches in thickness for each layer. Finally, a plated outer layer functions to allow reliable attachment of the lead frame to the circuit board using a variety of attachment methods. The outer layer typically is one or more of Tin, Tin/lead, Gold, Silver, and Palladium
The Cu, Cu alloy, Cu/Ni and/or Ni based alloys typically used as the base metal for the lead frame functions satisfactorily but is an expensive solution in the space for electronic components. It would be desirable to have a base material that provides the same functional properties, or better, of those metals conventionally used but at a lower cost.